Modem long haul optical networks transmit optical signals through thousands of kilometers of optical fiber. To compensate losses due to attenuation in the fiber, optical amplifiers are deployed at roughly 100-km intervals. An emerging amplification technology uses the Raman scattering process in the transmission fiber. By using a high power optical pump source at a frequency roughly 13 THz above the signal frequency, optical gain is provided for the signal, thereby replenishing the power lost to fiber attenuation.
The Raman gain process is dependant on polarization; this means that only light of like polarization will interact to give a net Raman gain Because the Raman process is polarization dependent, use of a polarized pump source can lead to polarization dependent gain, thereby changing the polarization state of the signal. Polarization mode dispersion and other polarization dependent losses in the system can then degrade the quality of the signal. Because the fiber can scramble the signal polarization, it is not feasible to anticipate the instantaneous polarization state of the signal. It is therefore desirable to use a pump source that is unpolarized. Then, no matter what polarization state the signal happens to be, even if it rotates, the pump beam will amplify it uniformly.
There are only a few techniques that are successful in depolarizing a highly polarized source. The one most employed now in industry is the combination of two polarized beams from two separate lasers of the same wavelength at orthogonal polarizations. To obtain a depolarized Raman pump beam 15, conventionally two individual orthogonally polarized pump laser beams 13 and 14 of similar wavelength are polarization multiplexed using a device called a polarization beam combiner 10, illustrated in FIG. 1A. When beams 13 and 14 are multiplexed together, the effective polarization becomes random, because lasers 11 and 12 are incoherent with each other. Multiplexed beam 15 becomes depolarized, because the vector sum of the amplitudes of beams 13 and 14 at any polarization angle is uniform A rigorous polarization measurement would show a zero degree of polarization.
In this setup the relative intensities of the two lasers 11 and 12 must be equal to maintain the desired state of depolarization. The technology being what it is today, this is not a problem. The unattractive aspect of this scheme is the cost and size. The number of wavelengths at which the signal is pumped drives amplifier performance, indirectly. In this scheme, every added wavelength requires two DFB lasers and a means to combine them, increasing the size of the package and more than doubling the cost per wavelength
Further, to be successful, this approach requires the use of two lasers of similar wavelength, which limits the ability of the pump laser to flatten the Raman gain profile. Additionally, the amplitudes of both source lasers must be actively controlled to be equal, or else one laser can dominate and actually increase the degree of linear polarization in the dominant direction.
The present invention is directed to a system and method of providing a depolarized optical beam, comprising a passive depolarizer incorporating an optical polarization beam splitter/combiner (PBC) having a first and a second input port and having a first and a second output port optically connected respectively with the first and said second input port. The PBC is configured such that light having a first linear polarization and entering the PBC through said first input port propagates out substantially entirely through the first output port. At least a portion of light entering through the first input port and having a second linear polarization orthogonal to the first linear polarization is switched out through the second output port. Similarly, light of the first linear polarization entering said PBC through the second input port propagates substantially entirely out through the second output port, and at least a portion of light entering through the second input port and having the second linear polarization is switched out through the first output port.
The passive depolarizer includes a loop back light transmission guide connecting the second input port with one output port of the PBC. This loop back light transmission guide is configured to have an optical length greater than the coherence length of light entering the PBC through the first input port.
In operation, a polarized input optical beam enters the PBC, where the first polarization component of the polarized input optical beam continues to propagate substantially entirely along a first optical path, whereas at least a portion of the second orthogonally polarized component switches onto a second optical path. The loop back transmission guide adds propagation delay in at least one of the first and second optical paths, such that the delay path has a length greater than the coherence length of the polarized input optical beam, giving rise to delayed and undelayed optical beam components, which are then incoherently recombined into a depolarized output optical beam.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.